1. Field of Invention
The invention relates to a process for treating polyurethanes, especially thermoplastically processable polyurethanes, to reduce the molecular weight thereof. More particularly, the invention is directed to processes for treating such polyurethanes as contain gel fractions, which comprise insoluble matter due to crosslinking, or excessive degree of polymerization, atypical polymermization, or like mechanisms, in order to eliminate or substantially reduce these gel fractions and/or to lower the molecular weight of the polymer.
Thermoplastically processable polyurethanes have been produced commercially for more than 20 years. The majority of products which are of industrial importance at the present are prepared by reaction of 4,4'-diphenylmethane diisocyanate with (1) an aliphatic polyesterdiol such as hydroxyl-terminated poly(1,4-butylene adipate) or an aliphatic polyetherdiol such as hydroxyl-terminated poly(oxytetramethylene) and (2) a diol chain extender such as 1,4-butanediol or 1,4-bis(2-hydroxyethoxy)benzene. Some of the commercially available polymers are predominantly hydroxyl-terminated, while others are prepared with a slight stoichiometric excess of isocyanate and are predominantly isocyanate-terminated. The latter polymers undergo further reaction upon elevated temperature post curing or aging in the presence of atmospheric moisture, and may become crosslinked. All of these polymers commonly are processed into shaped articles by means of thermoplastic processing techniques such as injection molding, extrusion, calendering, and blow molding. Many of the polymers, particularly those predominantly hydroxyl-terminated, are soluble in dipolar aprotic solvents such as tetrahydrofuran, dimethylformamide, cyclohexanone and dioxane, and as solutions in these and other solvents, find a variety of applications as coatings and adhesives.
For applications in solution, it is highly desirable if not essential that under given conditions of temperature, concentration of polymer and choice of solvent, the viscosity be within certain limits. Moreover, when thin films or coatings are to be prepared from the solutions, it is important that the polymer be entirely soluble, i.e., free of particles of crosslinked polymer commonly called "gel". It is difficult to produce consistently thermoplastic polyurethanes which are free of gel. Furthermore, it is equally difficult to achieve precise molecular weight control, and therefore solution viscosity control, in commercial manufacture of the thermoplastics. In addition, even nominally OH-terminated thermoplastic polyurethanes generally contain a residuum of NCO groups sufficient to cause marked increases in molecular weight within normal periods of storage required in commercial applications prior to final usage of the thermoplastic. Of those products which have viscosity specifications, in many cases the dilute solution viscosity may vary as much as by a factor of two, and even with this generous allowance for variation of the molecular weight, a considerable amount of material is produced which is out-of-specification.
In many coatings and adhesives applications, it would be desirable to reproduce solution viscosity of a given type of polyurethane thermoplastic to within .+-. 20 percent, or even less, of a predetermined level without changing the temperature, polymer concentration, or composition of the solvent. Up to the present time, this has not been generally possible. Furthermore, many lots of material offering the desired viscosity characteristics are rejected due to gel content. Also, where thin articles are produced, it is highly desirable to utilize polymers of low gel content. Nevertheless, it is typical to find a noticeable and objectionable amount of gel particles, for example, in polyurethane film manufactured from in-specification lots of polymer.
Also, the storage stability of many polyurethane thermoplastics, both as solids and in solution, has been a problem. If free isocyanate is present in the polymer, it is inherently unstable and tends to increase in molecular weight. This is particularly troublesome when the polymer is exposed to moisture. When the polymer is to undergo thermoplastic processing, such aging leads to less favorable processing characteristics and physical properties in the articles produced. In the case of solution applications, the viscosity usually increases, the content of gel may increase, and/or the polymer may become less soluble.